Means for opening and closing electrical circuits.



No. 833,178. PATENTED OCT. 16, 1906.

G. RENNERFELT.- MEANS FOR OPENING AND CLOSING ELECTRICAL CIRCUITS.

APPLICATION FILED JUNE 11, 1904.

s SHEETS-SHEET 1 WITNESSES; INVENTOR: 456M PATENTED OCT. 16, 1906.

C. RENNERFELT.

MEANS FOR OPENING AND CLOSING ELECTRICAL CIRCUITS.

APPLICATION FILED JUNE-11, 1904.

3 SHEETS-SHEET 2 By his Alto/nay,

PATENTED OCT. 16, 1906.

a sums-sum a G. RENNERFELT.

MEANS FOR OPENING AND CLOSING ELECTRICAL CIRCUITS.

APPLIOATION FILED JUNE 11, 1904.

' V x. ,IIIIIIIIIIIIIIIM INVENTOR: W W

By his Attorney WITNESSES:

UNITED STATES PATENT oFFIoE.

GUSTAF RENNERFELT, OF SORANTON, PENNSYLVANIA.

Specification of Letters Patent.

Patented Oct. 16, 1906.

A li ation filed June 11, 1904. Serial No. 212,127-

To all whom it may concern.-

Be it known that L'GUSTAF'RENNERFELT, a subject of the King of Swedenand Norway, residin at Scranton, county of Lackawanna, State 0Pennsylvania, have made a new and useful Invention in Means for Openingand Closing Electrical Circuits, of which the following is aspecification.

My invention is directed to improvements in means for openim and closingelectric circuits in general, and especially such circuits as includeelectric motors. I

The objects of my improvements are, first, to provide an electric switchor circuit-. breaker having a time-constant whereby the circuit may beopened if it is subjected to a prolonged overload, but may remain closedif the overload is of short duration; second, toprovide simple means foradjusting such a switch so that the same device may be used fordifferent loads; third, to provide means for magnetically blowing outthe are which is formed at this switch when it operates; fourth, toarrange the associated parts of the switch so that it will be simple ofconstruction and reliable in its operation; fifth, to provide means forclosing and opening said switch; sixth, to prevent sparking at auxiliarycontacts necessary for operating the switch, the arrangement being suchthat all of the sparking is confined to the contacts of the switch andalso such that waste of energy in the controlling solenoids or magnetsused inthe apparatus shall be reduced to a minimum; seventh, to providenovel means for obtaining proper pressure for good contact on thecontroller and to rovide that the same means shall extinguish sparkingon the contacts of the controller, and also that they shall serve toretain the controller in a certain position; eighth, to provide meansfor an automatic economical and reliable cooperation of the aforesaidcircuit breaker and controller in eneral, and especially so that nosparking s all occur in the auxiliary contacts necessary in thecombination, all sparking being confined to the contacts of thecircuit-breaker, and also so. that waste of energy in the solenoids ormagnets used in the a paratus may be reduced to a minimum.

his especial feature the reduction to a minimum of the energy wasted inthe magnetsis of particular value, not only from the point of view ofmere economy, in so far as it tends to lessen the cost of construction,the cost of operating the apparatus, and the cost of keeping it inrepair, but it is also of especial value in that it makes it possible tooperate the same magnets with either alternating or direct current.

My invention, which to some extent relates to improvements in ap aratusdescribed in United States Patent *0. 7 69,406, ranted to me September6, 1904,-will be fully understood by referring to the accompanyingdrawings, in which- Figure 1 is a plan view of'my novel switch. Fig. 2is a sectional view thereof, taken on the broken line A A, Figs. 1 and3, and as seen looking thereat from right to left in the direction ofthe arrows. Fig. 3 is a sectional view taken through Fig. 2 on thebroken line B B and as seen lookin thereat from left to right in thedirection 0 the arrows. Fig. 4 is a sectional view taken through Fig. 2on the line C C and as seen looking thereat from the top toward thebottom of the drawings in the direction of the arrows. Fig. 5 is a planview of the controller, Fig. 6 is a sectional view thereof, takenthrough Fig. 5 on the line D D and as seen looking thereat from left toright in the direction of the arrows. Fig. 7 is an end elevational viewas seen looking at Fig. 5 from the top toward the bottom of thedrawings, the controlling-magnet for regulating the pressure between thecontacting s ring and the contact-blocks of the rheostat eing shown insectional view; and Fig. 8 is a diagrammatic view showing theappllcation of my entire improvement 1n a system of motor-control and asconnected with a water-tank adapted to be filled by a pump driven by thecontrolling-motor, the same figure illustrating also novel means forautomatically disconnecting the motor from the circuit in the event ofoverload, due to any cause whatever, such disconnecting means 7 being inthe nature of a thermal device which is actuated by an abnormal flowofcurrent therethrough. r

Referring now to the drawings in detail, 1 represents theinsulating-base of the switch, supported in any preferred manner."

2 and 3 represent the conducting-terminals of the switch, and 4 theyielding contactarms thereof, made, preferably, of thin stri s ofsheet-copper and pivotally securedto the terminal 2 by a head 5, whichis connected in turn by a link 6 to an operating-handle 7, provided witha strong spiral spring 8, the tendency of which is to rotate the handle7 in the direction of the arrow, Fig. 2.

9 is a roller carried by the switch-head, and 10 is a locking-triggerfor locking the sw1tch in its closed position, said. trigger beingconnected to an msulating-base-l 1, carried by an armature-lever 12,located in front of the core 13-of a magnet 14.

15 is a retractile spring for the armaturelever 12, adapted to hold itin the position shown in Fig. 3, so that the trigger 10 will restagainst the roller 9 and lock the switch.

The plate 11' is of insulating material and also acts as a baffle lateor shieldto separate the two termina s of-the switch.

16 is arubber buffer adapted to make a cushion-stop for-a lug- 17,secured to the handle 7, when the switch is automatically released underthe influence of the strong spring 8. 4

18 is a stationary carbon contact secured to the terminal 3, and 19 is amovable carbon contact being carried by a yielding spring 20, secured tothe head of the. switch by screws,

,(see Figs. 2'and 4,) the function of said terminals-being to interruptthe circuit without danger of fusing the parts.

.2-1 is the stationary contact-block for the free end of the yieldingswitch-arm '4, said .block being of non-magnetic metal, preferablycopper.

The extensions of the pole-pieces of the ma et 14 are so constructed andlocated wit relation to the stationary contact 21 that the magneticlines of force flowing therefrom will tend to extinguish the are betweenthe movable and fixed terminals of the switch when the circuit isinterrupted.

22- is an automatically-movable contact,

made, preferably, of two thin strips of metal ofulifl'erent coefiicientsof expansion and sol-' dered together.

' 23 is a carbon pencil secured in aslit in the manner shown in Figs; 2and 3 and adapted .to contact with the stationary contacts 24 or adescription of the controller, 26 represents,

t e insulating-base, and 27 thecontrollerart m, pivotally securedthereto and carrying at its free end a sliding contact-plate 28,adapted-to contact at one end'agalnst aseries of rheostat segments orcontacts 29,

mounted upon the base 26 and connected to successive coils thereof, andat the other end against theundersurface of acontactlate 4 30, securedat its opposite ends direct y to the base, the arrangement being suchthat the free .end of. the controller-arm and the sliding contact-plate28, carried thereby, will move between the plate 30 and rest upon thesegments or contacts 29 as the arm is moved in either direction. I 4

31 is a weight secured, as in Fig. 5, to the end of a lever carried bythe controller-arm 27 and adapted to normally hold said controller-armin the off position, so that all of the rheostat-coils are in circuit.32 is an electromagnet secured at the outer end of the controller-arm 27and provided with an armature-lever 33, pivotally secured to its core,so that the short arm thereof shall support one end of the contact-plate28 and press it against theunder surface of the plate 30. The other-endof the contact-plate '28 is secured to the core of the magnet 32 and ispressed against the segments or contacts '29.

34 is a magnet secured to and carried by the controller-arm 27, and 35is an iron arm pivotally secured to the controlling-arm, 36 being asection of a-half-nut supported in a slot in the outer end of the arm 35and in such manner that-when the circuit is closed through the magnet 34the screw-threads ofv the half-nut will be brought into mechanicalconnection with the screw-threads of the screw 37 against the stress ofa leafspring 38, carried by the arm 35 and resting on the pole of themagnet 34.

Referring now to Fig. 8, 5O designates an pump 70. This pump isconnected to a reservoir or tank'71 by a pipe 72 and is arranged'to pumpwater through a pipe 73 into another water-tank 74. A float 39 isattached to the inner end of a lever, which is pivoted upon one edge ofthis water-tank 74, the outer end of said lever being provided with anelectric contact-plate adapted to make circuit between two contacts 42when the water has reached'lowlevel and at two additional contacts 43when the tank has been filled. As illustrated in the drawings, the wateris at low level and the pump-motor has just been connected in circuit bythe closing of the automatic switch. This closing is effected by thelifting-of'the core'44 by the solenoid 45, which when contacts 42 arebridged will receive current by the following circuit: from the positivecurrent-feeder 46 through the branch feeder 47, contacts 42, solenoid45, branch feeder 47 adjustable contact 48', thermostatic .device 49,branch feeder 50, armature 51 of the motor, to the negativecurrent-feeder 52. Consequently the solenoid 45 lifts the iron core 44,which is connected to the switch, so that the motorcircuit is closedfrom the branch feeder 50 I through the yielding switch-arm 4, thusbridging between the terminals 2 and 3 to y S I remains in circuit, sothat its armature pivreceive .current from contact-plate of therheostat, sliding contact-plate 28, coils series wit the shunt-field 53,and the mag-- net 34 will cause the sectional half-nut 36 to engage withthe screw 37, so that when the armature 51. of the motor starts thisscrew will be rotated in pro er direction'by reason of the belt 57 andpul ey 58, as illustrated in Fig. 8, to cause the upper end of thecontroller-arm 27 to move from right to left, thus gradually causing thecontact-plate 28 to,

successively move across the rheostat segments or contacts 29, and tothereby cut the resistance-coils out of the motor-armature 7 the motorstill continues to run and rotate the screw 37 the controller-arm willnot be advanced any farther. The magnet 32 still otally supported by thecore will cause one of the free ends of the contact-plate 28 to bearwith considerable friction against the under surface of the curved plate30, and additional friction will be exerted on the other end of thecontact-plate 28 between the core of the magnet 32 also performs anotherim-' .portant function in that by reason .of the magnetic field set upas it passes over the rheostat segments or contacts 29, which arepreferably of iron, the lines .of force blow out any spark which mayresult as the plate 28 passes from segment to segment. It will be notedthat the solenoid 45 is so connected that the current flowing through itwill be gradually decreased, as the starting resistance is cut out bythe rotation of the controller-arm 27 from right to left, and no currentwill flow through said solenoid when all of this resistance is out ofcircuit.

This feature constitutes. an important part.

of my invention and is made the sub ectmatter of claims hereinafter. Ifthis solenoid were so connected that current passed through it all thetime that the motor isrunring it would not be possible to open theswitch by such simple means as I provide,

and, furt rmore, considerable energy would be wasted by constantlymaintaining the solenoid in circuit, whereas as now constructed when thestarting resistance is all cut out of circuit both terminals of thesolenoid are connected. directl to the same currentmain. "If this'sofenoid were connected in 1 the ma such manner that it becamedenergized by the closing of contacts 2 and 3, it might possibly happenthat it would not act a sufficient length of time to give properpressure on the arm 7 and cause the locking of the switch,

which occurs, as is illustrated in Fig. 3, when ,tor-armature, so thatby reason of the variable drop of potential in the starting resistance Igain a result such that there will be sufiicient current through thesolenoid to close the switch with certainty and yet prevent waste ofenergy in the solenoid while the motor is running.

Suppose now that the'water has risen in the tank to its extreme limitand that the contact-plate 41, carried by the pivoted lever controlledby the float 39, has made contact with the two contacts 43, owing to therising of said float. When the circuit was interrupted.at the contacts42, no spark occurred because, as before stated, no current was flowingthrough the solenoid 45, there being no difference of potential betweenthe ends ofits coil. When the water has risen, therefore, to the highestlevel and the contacts 43 established, as stated, this will cause acurrent to flow through the branch circuit 50 from the positive main 46through the switch terminals 2 3 to the contact-plate 30, branch circuit59, contacts 43, branch circuit 59, coils of the magnet 14, branchcircuit 59, .to the negative main 52, thus energizing the magnet 14 andcausing the same to act upon the armature-lever, which controls theoper-. ation of the trigger 10, thereby releasing the roller 9 andallowing the spring 8, which controls-the opening of the switch, toquickly remove the free end of the yielding contactarm 4 from thestationary copper contact-bar 21 and also the carbon-term1nal19 from thestationary carbonterminal 18 (following later) opening the circuitwithout arcin When the main circuit is thus finally opene et 32 'isdenergized, and the counterweig t 31 restores the controller-arm 27 toits off positionthat is, the Iposition shown in the diagrammatic view,ig. 8and re- 1 places all of the starting resistance in themotor-circuit. During this movement of the controller-arm 27 there isonl little pressure and friction between the s 'ding contactplate 28,the rheostat-segments 29, and plate 30, since the magnet 32 has beendemagnetized.

If the motor should happen at any time to be subject-ed to a prolongedoverload caused, for instance, by a hot ournalbearing, the circuit willbe opened in the followingmanner: The thermostatic device 22 issocombined with the circuit that it is heated by the current, and when itreaches a certain temperature 1t establishes a contact 22 24,

I which, as clearly seen from the diagram, will have the same effect onthe switch as has the closing of the contacts 43, the switch will beopened, the motor will stop, and the startlng reslstance will bereinserted into the motor-circuit. The preferred form of thisthermostatic' device consists of a metallic strip and is included in thecircuit sot at itis traversed and heated by the main current or by adefinite amount of the main current. A

resistance 60 may be connected in shunt to' the strip 22 to take a partof this current. When the strip 22 is heated, it assumes a curved shape,and the contact 24 is so adjusted that it will be touched by the stripwhen it has reached a certain temperature;

When this happens, the magnet-winding 14 I will receive current throughthe following clr'cuitzfrom the plus main 46, through branch conductor50, terminal 2, yielding switch-arm 4, terminal 3, conductor 50, plate30, sliding contact-plate 28, conductor 50, thermostatic device 22,contact-point 24,

branch conductors 61 and 59, colls of the magnet 14, to main 52.Consequently the trigger 1-0 is withdrawn and the switch opened asbefore by theaction of the spring 8.

Upon separation 'of the contacts 22 24 there will be no spark there,since the circuit is already broken on the switch at 19 18. F Apush-button or similar device 62 may be arranged in a branch circuit 63,whereby ghe- 1s-- switch can at any time and from any tance be openedmanually, and it will be noted that the connection for this push-buttonis such that no spark will occur at its terminals, for reasons beforepointed out.

It will be observedthatthestrip 22. cannot make contact with theadjustable. contacts 48 24 at the same time, and the solenoid 45 andmagnet 14can therefore never be ener-.

gized through the contacts simultaneously.

This would happen if there were permanent,

connections from the solenoid 45 to the plus brush of the armature 51and if the contacts 42 were established while the strip 22 wassufliciently hot to touch the contact 24. Such a condition would preventthe locking of the switch by the trigger 10 and roller 9 and would causearcing of thecontacts 18 and 19. By connecting the solenoid 45, asshown, to t e contact 48 1 effect the result that after themotor-circuit has been automatically overloa again before some time haselapse This time will be long enough to allow the thern'ur opened, byreason of a prolonged it will not be automaticall closed static device22 to cool off sufliciently to establish the contacts 48 22, and duringthis time the motor, which is included in circuit with the strip 22,will also cool somewhat, so as to be better adapted to standthe-overload in case it should still exist when-the circuit isestablished. Should a hot bearing causethe overload, the bearin willhave a chance to cool, and the overloa may perhaps have disappeared whenthe circuit is closed again, due

.to the making of contact between the strip 22 current is ofshortduration, and its heating effect on the motor is not great unlessthe with, but also for the purpose startin is fre uently repeated.Neither will its lieating e ect be great on the thermo-- static device22, and for this reason this de vice will not act so as to open thecircuit for an overload of such .short duration as that.

due to starting. It will, however, open the c1rcu1t for an overloadwhich lasts' during some time-that is, such a length of time as willendanger the motor and will cause the temperature of the thermostaticdevice to rise to the predetermined limit-and it isevident that such anoverload may correspond to a current which is much smaller than thestarting-current. p p

It must be well understood that the. mode of operation of the switch orcircuit-breaker here described is very different from that of thewell-known types of circuit-breakers which open the circuit when thecurrent attains a certain strength and which when motor will not openthe circuit for an overload due to a current smaller than thestarting-current. It should also be noted with regard to the controllingdevices shown as assembled in diagrammatic view in Fig. 8 that theiroperation is performed in a very economica manner 1n view of the factthat there is no energy wasted in the magnetwindings 14, 34, and 45,which are tempura: rily energized only at the time when theyhave toperform actual mechanical work. The only winding which utilizescontinuously some amount of energy is 32, and as the force required ofthis magnet is not heavy it will be apparent that by properly designingthe air-gap and other details of the magnet 32 the amount of energythere wasted can be limited to a mere trifle. It might be added that incase of cessation of the voltage at the mains 46 and 52 the switch willnot be 0 ened; but since the magnet 32 will become eener- "used insimple combination with an electric gized the arm 27 will be brought toits off position, where all the starting resistance-will e reinsertedinto circuit with the motor.

When the voltage again appears and the motor starts up, the resistanceis again out out from the circuit.

Having thus described my invention, what I claim, and desire to secureby Letters Patent of the United States, is

1. In an electric circuit, the combination of a contact-block 21, aswitch-arm 4, a magnet for moving the arm into contact with the block,means for energizing the magnet and automatic means for graduallydecreasing to zero the current in said magnet.

2. In combination with an electric motor having an armature, a circuitfor the armature, a switch in saidcircuit, a latch for holding theswitch closed, a spring for opening the switch and a magnet arranged torelease said latch, said magnet having a winding in the action of thespring, automatic means for direct shunt with the motor-armature.

3. In an electric motor, the combination of a contact-block, aswitch-arm, a s ring arrangedto se arate the switch-arm cm thecontact-bloc a magnet arranged to move the arm into contact with theblock against the action of the spring, means for energizing the magnet,and means for gradually de-' creasing the current in the magnet to .zeroa latch for holding the arm against the contactblock, and a ma et forreleasing the latch.

4-. In an electric motor, thecombination of a contact-block, aswitch-arm,-a spring arranged to se arate the switch-armfrom thecontact-bloc a magnet arranged to move the arm into contact with theblock against energizing the ma net, and means for gradually decreasingt e current in the magnet tozero; a latch for holding the arm againstthe contact-block, a magnet forreleasing the latch, and means forautomatically energizing said releasing-magnet.

5. In an electric circuit, the combination of a motor, a startingresistance therefor, an electrically actuated starting-switch, a ma netarranged to close the starting-switc said magnet having a windingconnected .in shunt to the starting resistance. a

6. In combination with an electric motor,

' a device arranged to automatically stop the I I motor when said'deviceis heated to a predetermined degree and to automatically start the motorwhen cooled to a predetermined degree, the temperature of said devicebeing arranged to be proportionalto that of themotor. i 7. Incombination with an electric motor,

an automatic thermostatic device arranged to break the motor-circuitwhen current in said circuit is excessive, and to close said circuitafter an elapsed time, the length of said time depending upon the amountof said ex cess current.

8. In combination with an electric motor having an armature, astarting-switch therefor, a latch for holding the switch closed, a

spring for opening the switch, an' electromagnet arranged to release thelatch, said ma net having a winding connected in shunt to t e armatureof the-motor, and athermostat in series with the motor-armatureandarranged to be actuated by said currentand arranged to close the circuitthrough the magnet.

v9. In combinationwithan electricmotor a resistance in series withthemotor, a-starting-switch for the motor, a magnet arranged to closesaid starting-switch, said magnet having a winding connected in shunt tosaid resistance, and means for retaining the switch in its closedposition.

.10. Thecombination with a source of eleetrical supply, of a circuit, athermostat in said circuit arranged to be heated by current from saidsource of supply, a second electric circuit derivin current from saidsource of supply throug the thermostat, a contactpiece located in saidsecond circuit in close proximity to the thermostat, and means foradjusting the relative position of the thermostat and the contact-piece.

l 1. The combination with a source of elec-' trical sup ly, of atranslating device connected to'sai source ofsu ply, a thermostaticdevice in series with sald translating device,

a switch also in series with the translating device, a magnet having awinding andarranged, when energized, to open said switch,

a contact-piece located in the circuit of the too magnet-winding andadapted to come into electric contact with the thermostat when thelatter is heated to a predetermined decc, and means for adjusting thedistance etween the thermostat and the contactpiece. r

12. The combination with a source of electrical sup ly, of a translatingdevice connected to said source of electrical supply, a ther- TIO 'mostatic device in series withthe translating device, a switch also inseries with the translating device, a magnet having a winding andarranged, when ener 'zed, to close the switch, afcontact-piece ocated inthe circu1t of the magnet-winding and adapted .to come into electriccontact with the thermostat, and means for adujsti the distance betweenthe thermostat an the contactiece.

13; The combination with a source 0 electrical supply, a translatingdevice connected to said source of supply, a switchinseries with thetranslating device, a magnet having a wind' arranged, when energized, toclose the SWiI/Iil, said winding being also in series with thetranslating device, means for retairiing the switch in its closedposition, a thermostat inseries with the translating device, and meansoperated through the thermostat for opening the switch.

ITS

tor.

14. The combination of an electric motor having an varmature, twocontacts in series with the armature, a starting resistance, anelectromagnet in series with the armature and in-shunt to the startingresistance, means I actuated by the electromagnet for closing thecontacts, and means for retaining the contacts in closed position.

]5.- The combination of a motor, a circuit for the motor, means forclosing the circuit, a

thermostatic device dependent u on its tem-' perature for delaying thetime o the closing ofisaid circuit, and means dependent on itstemperature for opening the motor-circuit, the thermostatic device arraned to have its temperature proportional to that of the mo- 16. Thecombination of an electric motor,

I a starting-switch therefor, a magnet arranged,

magnet arranged, when energized, to openwhen energized, to close theswitch, a second the switch, circuits for the "magnets, and athennostatthrough which one or the other of said magnet-circuits may beenergized, said-thermostat being in series with the motor. 1 I

17. The combination of an electric motor, a starting-switch therefor, amagnet arranged,

' 'whenenergized, to close the switch, a second magnet arranged, whenenergized, to open the sw-1tch, c1rcu1ts for said magnets, an automatic'sw tch for closing one or the other of said ma 'net circuits, and athermostat in cirmotor, a tank hydraulically'connected with the pump, aswitch arranged to be-actua'ted by t 0 level pf water in saidtank toclose the circuit of said first magnet when the waterlevel is low and toclose the circuit of said second magnet when the water-level is high,

. and a thermostat in circuit withthe 'motor and arranged to controlsaid magnet-circuits. 19 In combination with an electrical source ofsupply, a circuit deriving current therefrom, a metallic stripincludedin said circuit and arranged to bend when heated by the current,a contact in close proximity to the strip, and means for adjustmg'thepdsiden of the contact relative to the strip, said contact beingarranged to be brought into or out of electric connection with the strip.upon

a movement of the strip, and means for avoiding breaking of any currentat said contact.

20. In combination with an electric circult, aswitch therefor, means forretalmng the switch in 1ts closed pos1t1on, a magnet,

and means coeperating with the. magnet to 6 open the switch when themagnet is energized, a thermostat included in the circuit, and throughwhich the magnet may be energized, and a resistance in shunt to the thermostat so arranged that a definite part of the currentin the circuitwill flow through the thermostat.

2 1 The combination of a switch, a spring arranged to open the switch, amagnet arranged to close the switch against the action of the sprin acircuit for said ma et, a contactswitc arranged to be close and openedin said circuit, means for holding the first switch closed, a magnet forreleaslng the holding means, a circuit for the releasing-magnet, acontact-switch arranged to be closed and opened inthe releasing-magnetcircuit, said circuits and contacts arranged to avoid the breaking ofany current at either of said contact-switches.

22. The combination with a source of electrical supply, of a thermostatarranged to be heated b a current from said source of supply, an eectric circuit derivin current from said source ofsu ply through t 1ethermostat, a contact-piece coated in the circuit in close proximity tothe thermostat, a carbon piece attached to the thermostat opposite tosaid contact-piece, and means for adjustin the relative position of thethermostat an the contact-piece.

23. The combination with a source of elec-- trical supply, of athermostat arranged to'be heated b current from said source of supply,an e ectric circuit derivin current from said source of supply througthe thermostat, a contact-piece coated in thecircuit in close proximitto the thermostat, and means for adjusting the relative position. of thethermostat and the contact-piece.

24. In combination with, an electrical source of supply, a circuitderiving current therefrom, a metallic strip included in said circuitand arranged to bend when heated by the current, a contact included inthe circmt in close proximity to the strip,- and means for adjusting theposition of the contact relative to the strip.

'25. In combinationwith an electric eircuit, an automatic thermostaticdevice in eluded in the circuit and arranged to open the circuit after atime limit when'the current in said circuit reaches a predeterminedamount, and to close the circuit after the

